1. Field of the Invention
This invention relates to an optical recording medium capable of recording and/or reproducing by light, particularly laser beam, more particularly to an optical recording medium having an organic dye thin film improved in optical characteristics and recording and reproducing characteristics.
2. Related Background Art
Generally speaking, an optical recording medium such as optical disc or optical card can record information at high density by forming optically detectable small pits of, for example, about 1 .mu.m on a thin recording layer provided on a substrate having spiral, circular, or straight line grooves. When converged laser beam is scanned on the surface of the recording layer, the recording layer absorbs the laser energy to form optically detectable pits, whereby information is written.
For example, according to the heat mode recording system, the recording layer absorbs heat energy, whereby small concave pits can be formed by evaporation or melting at that site. According to another heat mode recording system, by absorption of the laser energy irradiated, pits having optically detectable density difference can be formed at that site.
The information recorded in the optical recording medium is detected by scanning a laser beam along the track and reading the optical change between the portion where the pit is formed and the portion where no pit is formed. For example, in the case of optical disc, the laser beam is scanned along the track, and the photoenergy reflected by the disc is monitored by a photodetector. The portion where no pit is formed deflects sufficiently the laser beam and the output of the photodetector becomes greater. On the other hand, at the portion where the pit is formed, deflection becomes lower to make the output of the photodetector smaller.
As the recording layer of such optical recording medium, inorganic compound films such as metal thin films of Te, Bi, Sb, etc. or chalcogenide type amorphous thin films, etc. have been primarily used. However, these thin films have various drawbacks such that it is difficult to control the reflectance relative to laser beam and that they are liable to be oxidized by the influence of temperature, humidity, etc.
On the other hand, studies of optical recording medium comprising an organic dye thin film capable of changing optical properties by use of photoenergy have been actively done as shown in Japanese Laid-Open Patent Application No. 219090/1983. Use of an organic dye can make it possible to form easily a thin film having absorption ability corresponding to the oscillated wavelength of the laser beam, and a thin film of a dye alone has a particular reflection ability, whereby reflected light necessary for reproduction can be obtained. Further, an organic dye thin film can be formed by dissolving an organic dye in a solvent and coating it, thus being also excellent in bulk productivity. As such solvent, in ordinary cases, it may be preferable to use a good solvent for the organic dye such as dichloroethane, etc. However, since such a solvent will corrode the resin substrate frequently used as the substrate for optical card or optical disc such as polymethyl methacrylate or polycarbonate substrate, they cannot be coated directly on such substrate. Accordingly, for direct coating on a resin substrate, solvents which will not corrode substrates are selected, and these are generally poor solvents for organic dyes, and therefore even when coated on substrates, it is impossible to obtain an organic dye thin film which has necessary film thickness and uniformity to be used as the recording layer. However, the poor solvent as mentioned here refers to a solvent which dissolves only 1 wt. % or less of an organic dye, and examples of such solvent may include alcohols, ethers, esters, aromatics, aliphatic hydrocarbons, etc.
When these solvents are used, an organic dye thin film with necessary thickness can be prepared for the recording layer by coating a dispersion of the organic dye. However, the thin film recording layer thus obtained is not uniform, whereby there has been involved the problem that the noise level of the optical information recording medium having this recording layer is increased to deteriorate C/N ratio.